[lime-users] Backup power supply with network alert

Has anyone set up a system that does this: When the public power grid cuts off, a backup power supply gives power to the node and a message is sent to the network saying "Main power cut off, switched to backup power." Then, when the power comes back on, a message is sent saying "Main power returned." I imagine a RaspberryPi could do this, but I don't know how, and I wonder if there are less expensive ways to do it. The messages could be sent to the gateway node in our community and stored on that router, or email or something else. This would help us because the power goes out about two times per week at our gateway node, but the only symptom we see is that our internet connection cuts off, and we don't know if it's because of power outage or ISP problems. We want to know, because this way we can make official complaints about the quality of service to the ISP, and we can decide how much backup battery power we need in order to enable continuous connectivity.